Passive pseudorandom noise (PRN) ranging systems such as the United States' Global Positioning System (GPS) and the Union of Socialist Republics' Global Navigation System (GLONASS) allow precise determination of latitude, longitude, elevation and time. A PRN ranging system receiver accomplishes this by using time difference of arrival and Doppler measurement techniques on precisely-timed signals transmitted by orbiting satellites. Because only the satellites transmit, the need for two-way communications is avoided, and an infinite number of receivers may be served simultaneously.
However, the transmitted signal must contain a number of components in order for the receivers to extract the requisite information; Thus each satellite transmits on at least one carrier frequencies. Each carrier is modulated with low frequency (typically 50 Hz) digital data which consists of information such as the satellite's ephemeris, (i.e. position), current time of day, and system status information. The carrier is further modulated with one or more high frequency unique pseudorandom noise (PRN) codes.
A PRN receiver thus receives a composite signal consisting of one or more of the signals transmitted by the satellites within view, that is within a direct line-of-sight, as well as noise and any interfering signals. Because the signals transmitted by different satellites use unique PRN codes or unique frequencies, the receiver may separate the signals from different satellites using code-division multiple access (CDMA) or frequency division multiple access (FDMA) techniques. The PRN codes also provide a mechanism to precisely determine the signal transmission time from each satellite. By determining the transmission time from at least four satellites, and knowing each satellite's ephemeris and approximate time of day information, the receiver's three dimensional position, velocity and precise time of day can be calculated.
For more information on the format of the GPS CDMA system signals, see "Interface Control Document ICD-GPS-200, Sept. 26, 1984", published by Rockwell International Corporation, Satellite Systems Division, Downey, Calif. 90241.
For more information on the format of the GLONASS system signals, see "The GLONASS System Technical Characteristics and Performance", Working Paper, Special Committee on Future Air Navigation Systems (FANS), International Civil Aviation Organization (ICAO), Fourth Meeting, Montreal, Quebec, Canada, May 2-20, 1988.
A number of difficulties exist with present-day PRN receivers. One such problem concerns accurate tracking of the received composite signal. Another is determining and correcting carrier phase lock loop cycle slipping. Another is determining and correcting ionospheric divergence. The typical PRN receiver includes a downconverter and a mixer. At the downconverter, the input radio frequency (RF) signal from the antenna is amplified, filtered, and downconverted to an IF frequency by mixing it with a locally generated carrier reference signal. The decoder, typically consisting of a mixer positioned either before or after the downconverter multiplies the incoming signal with a locally generated PRN code reference signal. If the locally generated PRN code is properly correlated with that of the incoming signal, the digital data results.
In either implementation, however, the circuits required to perform the frequency and phase discrimination necessary to accurately generate the local carrier and PRN code reference signals are complex and costly, mainly because the composite signal ideally has a power level of 16 decibels (dB) below thermal noise. These circuits may also introduce errors due to component mismatches when the signals from several satellites are processed to calculate the receiver's position.
In addition, given the low level of received composite signal, CDMA receivers are typically quite susceptible to corruption by interfering continuous wave (CW) signals.
For high accuracy applications, atmospheric distortion of the transmitted signals must also be corrected. The principal source of such distortion is the ionosphere, which manifests itself as a delay of the PRN code phase and advance of the carrier phase. Since the amount of ionospheric distortion is a function of the carrier frequency and the total electron count along the path of the signal, the discrepancy in delay between two signals transmitted at different carrier frequencies from a single satellite may be used to determine an absolute correction factor. However, this absolute divergence correction requires twice the hardware to demodulate and decode both frequencies simultaneously. Furthermore, in some systems, such as GPS, access to the PRN code on the second frequency is limited. In certain systems, a relative divergence between the two frequencies can be determined by tracking the phase difference in carrier frequencies, without first decoding the second carrier, by simply squaring the received carrier signal. However, either approach requires radio frequency (RF) circuitry which at least partially processes two RF signals.